抄録
In the aseismic design of framed structures, design seismic loadings are usually given as equivalent horizontal forces acting on the structure at the same time. On the other hand, during an earthquake, the maximum shear force acting at each story does not necessarily occur simultaneously, therefore some of important effects of dynamic loads upon the response of the structure cannot be explained without resorting to a dynamic response analysis. This paper proposed to use the second joint moments of story shear forces in order to express the vertical distribution of seismic loads including the correlations between story shear forces. The distribution was determined based on the results of a modal analysis of uniform shear beam. Using this seismic load distribution, a method was devised to predict a collapse mechnism that could form with the greatest probability in a framed structure under earthquake excitations. Moreover, we made the assumption that the above mentioned mechanism formed during catastrophic earthquakes and according to the mechanism plastic deformations continued to glow only in one direction. With these assumptions, the energy absorbing capacity of a framed structure until collapse was obtained analytically. Further, as an application of the results discussed herein, a method to proportion members was shown in order that the material consumption is a minimum and that the framed structure satisfies following two different criteria : first, it behaves elastically during moderate earthquakes; second, the structure does not collapse even under catastrophic earthquakes.
